Voltage setpoint error reduction

ABSTRACT

A precision voltage regulator comprises a three terminal regulator coupled to a voltage divider. The voltage divider has two composite resistors, each of which comprises a plurality of matched value resistors fabricated on a common substrate, mixed in series and parallel configurations. The resultant voltage divider produces a wide range of divider ratios, while preserving a divider ratio which is independent of temperature and tolerance effects.

FIELD OF THE INVENTION

This invention is directed to the class of power supplies havingprecision output voltages. One particular class of precision outputpower supplies may further utilize a precision voltage divider incombination with a precision voltage reference for the generation of anerror voltage. Different combinations of precision voltage dividerratios may be formed from a single resistor network comprising aplurality of resistors of equal value.

BACKGROUND OF THE INVENTION

Precision power supplies have applications in electronic equipmenthaving narrow tolerance output voltages. It is not uncommon in state ofthe art electronic equipment to have requirements for output voltagetolerances of less than +/-1%. This disclosure is drawn to the class ofsuch power supplies producing a closely regulated output.

The use of matched resistors as voltage dividers is shown in U.S. Pat.No. 4,161,742 by Cheek et al. This reference shows the use of matchedratio resistors in stepped values combined with a detector for findingcross connections in a wiring harness. The details of fabricatingmatched resistors in semiconductor devices is discussed in U.S. Pat. No.4,565,000 by Brokaw. The use of matched resistors for increased accuracyof an A/D converter in U.S. Pat. No. 4,769,628 by Hellerman and in U.S.Pat. No. 5,648,780 by Neidorff.

SUMMARY OF THE INVENTION

The present invention enables a power supply to generate a precisionoutput voltage using a voltage divider formed from a resistor networkwherein the individual resistors have the same value, and because theywere fabricated on the same substrate in close proximity to each other,are intrinsically closely matched to each other in temperaturecoefficient. Therefore, a first object of the invention is a powersupply having a precision nominal output voltage. A second object of theinvention is a power supply having a nominal output voltage which isinsensitive to changes in temperature. A third object of the inventionis a resistor divider having a precision divider ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the schematic diagram of a prior art power supply.

FIG. 2 is the schematic diagram of a voltage divider formed from aresistor network.

FIG. 3 is the top view of the voltage divider network of FIG. 2.

FIG. 4 is the schematic diagram of a voltage divider formed from aparallel network.

FIG. 5 is the schematic diagram of a voltage divider formed from aseries network.

FIG. 6 is the schematic diagram of a voltage divider formed from acombination of series and parallel networks.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a prior art voltage regulator, using as an example LinearTechnology LT317 adjustable voltage regulator. The essential elements ofthe power supply are adjustable regulator 12, which has an input 12in,an output 12out, and an adjustment input 12adj. The adjustable regulatoroutput 12out is coupled to the load 30, and a voltage divider 28. Thevoltage divider 28 has an output terminal 28out, a common terminal28com, and an adjustment terminal 28adj, which is connected to theadjustable regulator adjustment input 12adj. The adjustable regulator 12further comprises a pass transistor 22, a precision voltage reference20, and a precision differential amplifier 14 having an inverting input16, and a non-inverting input 18. The equations of operation foradjustable regulator 22 are as follows:

    Vo=V16+Vref

where

Vo is the output voltage at terminal 12out

Vref is the reference voltage produced by 20

V16 is the voltage present at terminal 12adj

In practice, the DC gain of amplifier 14 is usually 10⁶, and the inputoffset voltage of amplifier 14 is less than 2 mV, and precision voltagereference 20 produces 1.250±25 mV. The input bias current to theadjustment pin is 100 uA. Using these values for the Linear TechnologyLT317A produces the following output equation: ##EQU1##

FIG. 2 shows the details of the voltage divider. Resistor network 28 isa 6 resistor network where each resistor has the same value. An exampleof this network is part number CAT16-103-J4, manufactured by Bourns Inc.This is a thick film metallized resistor fabricated by depositing a thinfilm of metal or a resistive material on a ceramic substrate. Becausethe individual resistors 28a, b, c, d, e, and f are in close proximityto each other and share the same resistance coefficient for temperature,the individual resistors tend to have the same value of resistance bothin nominal value, as well as with tracking of temperature.

FIG. 2 shows resistors 28b through 28f connected in parallel, whichproduces a composite resistor 24 equal to R28/5. The divider coefficientfrom 28out to 28adj is then 1/1.2=0.833333. In general, given nresistors in a package, one of which is used for resistor 28a, and n-1are in parallel for resistors 28b,c, and so on, the divider ratio is1/n. So for the example of FIG. 2, n=6, and the divider ratio is 1/6.For the same case where 2 resistors are placed in parallel for 28a, andthe remainder n-2 of the resistors are in parallel for resistor 24, asin FIG. 4, the divider ratio is 2/n. In the general case of a resistornetwork having n resistors, m in parallel to form resistor 26 and n-m inparallel to form resistor 24, the divider ratio is m/n. Thus, anydivider ratio of m/n may be generated using a resistor pack having nresistors. A sensitivity analysis of the resistor sensitivities showsthat if temperature causes the resistor pack to increase in temperatureby some temperature ΔT, and the resistor pack has a temperaturecoefficient of 200 ppm per degree Celcius, the change in resistance ofΔT*200*10⁻⁶ is applied to each of m and n of the above ratio. Hence, thefractional increase in effective resistance of the ratio m/n does notchange, and the divider ratio stays fixed despite the increasedresistance of each of m and n.

FIG. 3 shows the top view of a resistor pack wired as shown in theschematic of FIG. 2. Connections are made to resistor 28a in the middleto better thermally couple resistor 28a to the surrounding resistors28b-f. Terminals 28adj, 28out, and 28com are shown corresponding to thematching terminals of the same name in FIG. 2.

FIG. 4 shows an alternative embodiment of the voltage divider 28, shownin the figure as divider 32, having individual resistors 32a-e formedfrom the network. Resistors 32a and 32b are placed in parallel in thisembodiment.

FIG. 5 shows another altermative embodiment of the voltage divider 28,shown in the figure as divider 34, having individual resistors placed inseries.

FIG. 6 shows another alternative embodiment of the voltage divider 28,shown in the figure as divider 36, wherein the resistance is furthertrimmed by the series combination 36c and 36d, which are in parallel toresistors 36e and 36f.

Careful selection topology configurations of matched resistors formingresistor 24 and 26 of FIG. 1 will yield many precision divider ratiovalues. It should be clear to one skilled in the art that while theforegoing invention specifies for examplar purposes resistor networkscomprising 6 resistors, the invention clearly scales to n resistorswithout limit. Similarly, not all combinations of series and parallelresistor topologies are described in FIGS. 2-6. Nevertheless, it isclear to one skilled in the art that there is no limitation to thecombinations of resistor networks that can be made using series andparallel networks, including the use of resistors which span the threenodes "out", "adj", and "com" of divider 28 in arbitrary ways.

I claim:
 1. A precision power supply having an input terminal, an outputterminal, and a common terminal, said power supply comprising:anadjustable regulator having a voltage input coupled to said power supplyinput terminal, a voltage output coupled to said power supply outputterminal, and an adjustment input, said adjustable regulator producingan output terminal voltage equal to said adjustment input voltage addedto a reference voltage, a voltage divider having a divider outputcoupled to said power supply output terminal, a divider common coupledto said power supply common terminal, and a divider adjustment outputcoupled to said adjustable regulator adjustment input; said voltagedivider comprising a plurality n of individual resistors of equal valuesfabricated on a common substrate having a center wherein n/2 resistorsare on one side of said center, and n/2 resistors are on the other sideof said center, m of said resistors connected in parallel to form afirst resistor connected between said voltage divider output and saidvoltage divider adjustment output, n-m of said resistors connected inparallel to form a second resistor connected between said voltagedivider adjustment output and said divider commons; said m firstresistors and said n-m second resistors distributed equally on each sideof said center and thermally coupled to each other such that thermalgradients across the area of said m first resistors and said n-m secondresistors have minimum effect on the divider ratio formed by dividingsaid divider adjustment output by said divider output, wherein said n isan integer larger than said m.
 2. The regulator of claim 1 where saidadjustable regulator is a monolithic integrated circuit.
 3. Theregulator of claim 2 wherein said reference voltage comprises a bandgapvoltage reference.
 4. The regulator of claim 3 where said resistordivider comprises a thick film resistor network.
 5. The regulator ofclaims 1, 2, 3, or 4 wherein said m resistors are interleaved with saidn-m resistors.
 6. A precision power supply having an input terminal, anoutput terminal, and a common terminal, said power supply comprising:anadjustable regulator having a voltage input coupled to said power supplyinput terminal, a voltage output coupled to said power supply outputterminal, and an adjustment input, said adjustable regulator producingan output terminal voltage equal to said adjustment input voltage addedto a reference voltage, a voltage divider having a divider outputcoupled to said power supply output terminal, a divider common coupledto said power supply common terminal, and a divider adjustment outputcoupled to said adjustable regulator adjustment input; said voltagedivider comprising a plurality n of individual resistors of equal valuesfabricated on a common substrate having a center wherein n/2 resistorsare on one side of said center, and n/2 resistors are on the other sideof said center, m of said resistors connected in series to form a firstresistor connected between said voltage divider output and said voltagedivider adjustment output, n-m of said resistors connected in series toform a second resistor connected between said voltage divider adjustmentoutput and said divider common, said m first resistors and said n-msecond resistors distributed equally on each side of said center andthermally coupled to each other such that thermal gradients across thearea of said m first resistors and said n-m second resistors haveminimum effect on the ratio formed by dividing said divider adjustmentoutput by said voltage divider output, wherein said n is an integerlarger than said m.
 7. The regulator of claim 6 where said adjustableregulator is a monolithic integrated circuit.
 8. The regulator of claim7 wherein said reference voltage comprises a bandgap voltage reference.9. The regulator of claim 8 where said resistor divider comprises athick film resistor network.
 10. The regulator of claims 6, 7, 8, or 9wherein said m resistors are interleaved with said n-m resistors.
 11. Aprecision power supply having an input terminal, an output terminal, anda common terminal, said power supply comprising:an adjustable regulatorhaving a voltage input coupled to said power supply input terminal, avoltage output coupled to said power supply output terminal, and anadjustment input, said adjustable regulator producing an output terminalvoltage equal to said adjustment input voltage added to a referencevoltage, a voltage divider having a divider output coupled to said powersupply output terminal, a divider common coupled to said power supplycommon terminal, and a divider adjustment output coupled to saidadjustable regulator adjustment input; said voltage divider comprising aplurality n of individual resistors of equal values fabricated on acommon substrate having a center wherein n/2 resistors are on one sideof said center, and n/2 resistors are on the other side of said center,m of said resistors connected in a combination of parallel and series toform a first resistor connected between said voltage divider output andsaid voltage divider adjustment output, n-m of said resistors connectedin a combination of parallel and series to form a second resistorconnected between said voltage adjustment output and said dividercommon, said m first resistors and said n-m second resistors distributedequally on each side of said center and thermally coupled to each othersuch that thermal gradients across the area of said m first resistorsand said n-m second resistors have minimum effect on the ratio formed bydividing said divider adjustment output by said voltage divider output,wherein said n is an integer larger than said m.
 12. The regulator ofclaim 11 where said adjustable regulator is a monolithic integratedcircuit.
 13. The regulator of claim 12 wherein said reference voltagecomprises a bandgap voltage reference.
 14. The regulator of claim 13where said resistor divider comprises a thick film resistor network. 15.The regulator of claims 11, 12, 13, or 14 wherein said m resistors areinterleaved with said n-m resistors.
 16. A voltage divider having aninput, an output, and a common, said voltage divider having a firstresistor connected between said input and said output, and a secondresistor connected between said output and said common, said first andsecond resistors comprising a plurality n of individual resistors ofequal values fabricated on a common substrate having a center whereinn/2 resistors are on one side of said center, and n/2 resistors are onthe other side of said center, m of said resistors connected in parallelto form said first resistor, n-m of said resistors connected in parallelto form said second resistor,said m first resistors and said n-m secondresistors distributed equally on each side of said center and thermallycoupled to each other such that thermal gradients across the area ofsaid m first resistors and said n-m second resistors have minimum effecton the ratio formed by dividing said divider adjustment output by saidvoltage divider output, wherein said n is an integer larger than said m.17. A voltage divider having an input, an output, and a common, saidvoltage divider having a first resistor connected between said input andsaid output, and a second resistor connected between said output andsaid common, said first and second resistors comprising a plurality n ofindividual resistors of equal values fabricated on a common substratehaving a center wherein n/2 resistors are on one side of said center,and n/2 resistors are on the other side of said center, m of saidresistors connected in series or parallel combinations to form saidfirst resistor, n-m of said resistors connected in series or parallelcombinations to form said second resistor,said m first resistors andsaid n-m second resistors distributed equally on each side of saidcenter and thermally coupled to each other such that thermal gradientsacross the area of said m first resistors and said n-m second resistorshave minimum effect on the ratio formed by dividing said divideradjustment output by said voltage divider output, wherein said n is aninteger larger than said m.